Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
FIG. 1 illustrates a conventional front side illuminated CIS 100. The front side of CIS 100 is the side of substrate 105 upon which the pixel circuitry is disposed and over which metal stack 110 for redistributing signals is formed. The metal layers (e.g., metal layer M1 and M2) are patterned in such a manner as to create an optical passage through which light incident on the front side CIS 100 can reach the photosensitive or photodiode (“PD”) region 115. To implement a color CIS, the front side further includes a color filter layer 120 disposed under a microlens 125. Microlens 125 aids in focusing the light onto PD region 115.
CIS 100 includes pixel circuitry 130 disposed adjacent to PD region 115 within a P doped well. Pixel circuitry 130 provides a variety of functionality for regular operation of CIS 100. For example, pixel circuitry 130 may include circuitry to commence acquisition of an image charge within PD region 115, to reset the image charge accumulated within PD region 115 to ready CIS 100 for the next image, or to transfer out the image data acquired by CIS 100.
FIG. 2 illustrates two neighboring CIS pixels 100 formed within a P-epitaxial (“epi”) layer 140 disposed over a P+ substrate 105. When a photo-generated charge carrier is formed shallow within a pixel (e.g., charge carrier 150), it experiences a strong upward attractive force (shown by the arrows) towards PD region 115, due to the depletion region or P-N junction between the PD and the surrounding epitaxial layer. When a photo-generated charge carrier is formed deeper within a pixel (e.g., charge carrier 155), it initially experiences a weaker upward repulsive force due to the dopant gradient at the junction between the P− epi layer 140 and the P+ substrate 105.
Crosstalk is a serious problem in image sensors. There are three components to crosstalk: a) electrical crosstalk, b) optical crosstalk, and c) spectral crosstalk. Spectral crosstalk is caused by the diffraction and/or scattering of light off of metal lines and at interfaces between the dielectric layers within metal stack 110. Spectral crosstalk results from the finite (nonzero) transmittance of color filter 120 to wavelengths outside its target pass band, such as the finite transmittance of green and blue wavelengths through a red filter.
One form of electrical crosstalk is lateral drift of photo-generated charge carriers created deep in the semiconductor epitaxial layers (e.g., charge carrier 155). As these photo-generated charge carriers rise, they can drift laterally and end up collected in the PD region of a neighboring pixel. Blooming is another form of electrical crosstalk characterized by the lateral diffusion of charge carriers when a PD region becomes full or saturated with charge carriers. Blooming is most commonly experienced in high luminous environments. Photo carriers that are generated near a saturated PD region 115 will not be collected and therefore remain free to diffuse laterally into a neighboring pixel. Blooming results in the blurring of edges in still images and streaking in moving images. Both forms of electrical crosstalk are due to charge carriers generated in one pixel being collected by a neighboring pixel.